1. Field of the Invention
This invention relates to an exposure adjusting device such as a diaphragm or a diaphragm-shutter device adapted for use in a lens-incorporating camera, an interchangeable lens, etc.
2. Description of the Related Art
The non-interchangeable lens type 35 mm cameras are generally called compact cameras. The camera of this type is provided with an exposure device called a lens shutter device which is arranged to perform combined functions as a shutter and a diaphragm. The lens shutter devices used for the compact cameras manufactured these days are arranged to be of a direct electromagnetically driving type. In the lens shutter device of this type, a sector ring which carries shutter blades, i.e. diaphragm blades, is arranged to serve as a rotor for the electromagnetic driving device, i.e. a motor. Meanwhile, there has been proposed, although not practicalized as yet, a lens shutter device of the kind arranged to rotate the sector ring by means of a cylindrical or ring-shaped stepping motor including a ring-shaped rotor having about the same diameter as a lens barrel in use. For example, Japanese Patent application Laid-Open No. SHO 58-17428 discloses a lens shutter device of this kind.
These known lens shutter devices are arranged to have the sector ring which is carrying the diaphragm or shutter blades driven by a motor connected directly to the ring. This arrangement enables the sector ring to be rotated to a desired degree of angle to a desired position without recourse to any complex mechanical device. They are, therefore, not only small in size and light in weight but also suited for electronic control arrangement.
Meanwhile, in the case of the single-lens reflex cameras of the kind using an interchangeable lens of large diameter, a focal plane shutter is disposed within a camera body while a diaphragm device is disposed within a lens barrel, that is, within the interchangeable lens. The diaphragm device is arranged to be mechanically driven through an interlocking lever by a diaphragm driving device which is disposed within the camera body. The conventional single-lens reflex camera which is arranged in this manner necessitates a complex mechanical arrangement not only for the diaphragm driving device within the camera body but also for the internal devices of the lens barrel in association with the diaphragm device including the interlocking lever. This has increased the weight and size of the whole camera including the lens barrel. Besides, since the operating force on the diaphragm device must be transmitted through a complex mechanical force transmission device, the diaphragm device has had insufficient responsivity to control actions inadequate for electronic control.
Therefore, it has been necessary for conventional single-lens reflex cameras to reduce the weight of the whole camera by improving the diaphragm device and to have the device arranged to be capable of operating at high speed with better control responsivity. In order to obtain such a diaphragm device that is capable of meeting these requirements for a single-lens reflex camera, the device is preferably arranged to be driven directly by a motor in the same manner as in the case of the diaphragm device adapted for a compact camera.
However, the electro-magnetic driving device for the lens shutter of the conventional compact camera includes, as mentioned in the foregoing, a sector ring which carries the diaphragm blades and serves as a rotor by itself (or through an annular disc arranged to have the same diameter as the sector ring and to be in one unified body with the latter). Therefore, the rotor itself has a very large inertia. Assuming that the diameter of a rotating body is D and the gravitational acceleration thereof G, the inertial mass of the rotating body is proportional to GD.sup.2. The inertial mass of the rotor of the electro-magnetic driving device which uses the sector ring of large diameter D as the rotor thereof is, therefore, very large. The driving device thus has poor control responsivity. Besides, a ring-shaped coil which is used for the stator of the driving device has an effective diameter equal to the diameter of the sector ring. It is thus impossible to obtain a high ampere turn as the number of turns cannot be increased. Therefore, while the driving device is usable as a diaphragm driving motor for the compact camera having a small aperture lens, it has been hardly possible to use the arrangement of it as a motor for a large aperture diaphragm device of a single-lens reflex camera. Assuming that a motor is designed for a diaphragm device of a large aperture to have the same structural arrangement as the above stated electro-magnetic driving device, the magnetomotive force does not increase in proportion to the great increase of the inertial mass of the rotor. Therefore, it becomes difficult to start the rotor. The device tends to come out of step and the rotor then would fail to rotate and remain vibrating in its halting position.
It is thus apparent that the motor driven diaphragm device for a large aperture lens cannot be arranged in the same manner as the conventional lens shutter device adapted for a compact camera.
The present applicant has previously filed U.S. Pat. Nos. 4,763,150 and 4,695,145 for diaphragm devices having large apertures. The same applicant also has U.S. Pat. No. 4,681,417 for the arrangement of a maximum aperture detection switch. Further, the same applicant has also filed U.S. Pat. application Ser. No. 035,453 (Apr. 7, 1987) for a motor driven type diaphragm device which is of the same kind as the device of the present invention.